Exploding bridgewire coating



May 26, 1964 s. ZEMAN 3,134,329

. EXPLODING BRIDGEWIRE COATING Filed May 10, 1962 SAMUEL ZEMAN INVENTOR.

BY WW ATTORNEY 3,134,329 r EXPLODING BRIDGEWIRE COATING ,Samuel Zeman, Huntsville, Ala, assignor to: Thiokol Chemical Corporation, Bristol, Pa., a corporation of Delaware Filed May 10, 1962, Ser. No. 193,729 18 Claims. (Cl. 102-28) This invention relates to electrically actuated deflagrating squibs, and more particularly to electrically actuated deflagrating squibs of the exploding bridgewire type. Deflagrating squibs of the exploding bridgewire type have found increasing use, particularly as a means of initiating the ignition of solid-propellant rocket charges,

because of their resistance to accidental actuation. Very high values of electrical energy (2 0,000,000 ergs) must be applied to the bridgewire in a very short time (less than 4 microseconds) to cause it to develop the characteristic high temperature and high pressure shock wave phenomenon known as bridgewire explosion. The energy of this explosion is transmitted to a pyrotechnic charge, causing it to ignite; Accidental subjection of the bridgewire to less intense or less concentrated electrical energy will not ordinarily explodefthe bridgewire F and actuate the squib. Further, the pyrotechnic compositions used are such that they will not, normally, ignite when the bridgewire is heated by relatively low intensity electrical current, accidentally applied directly 'or induced by extraneous sources of electrical energy. 1, Although recent prior art exploding bridgewire (EBW) fsquibs are more resistant to accidental actuation than prior art ignition devices, there are a number of circumstances under which theymay he accidentally actuated.

If subjected to electrical energies slightly below those required to -explode the bridgewire, the bridgewire may be heated to incandescence or to its fusion point, accidentally actuating the squib. Again, the bridgewire -may be'subjected to prolonged electrical heating and, although not heated to incandescence, may raise the temperature of the pyrotechnic charge to the point at which it. will undergo, auto-ignition.

Structural and/ or chemical alterations may be made to overcome such problems, but the more resistant to accidental actuation an EBW squib is made, the more resistant to intentional actuation 'it may become. In addition, the delay time, or time to full ignition of the pyrotechnic charge following actuation, may be seriously affected. In extreme instances, a squib may be made so safe that actuation is unpredictable or problematical.

In spite of such problems, however, service requirements constantly call for EBW squibs with increased resistance to accidental actuation without increased delay time.

It is, therefore, an object of the present invention to provide an improved EBW squib which has greater resistance to accidental actuation. It is a further object of of the invention is advantageously employed. Such a squib comprises a body 1, usually of metal, with a pair of electrodes 2 for contact with a mating connector (not shown) and electrically insulated from each other and from the squib body 1 by insulator 3, composed of glass,

United States Patent Ofilice [oration of the Water, is PETN and 50% sodium plastic, or similar electrically insulating material. An additional electrical insulator 4 may also be employed. The pair of electrodes 2 isbridged within the squib by a bridgewire 5, usually of noble metal or noble metal alloy. A coating 6, employing a composition of the present invention, is applied to the bridgewire 5, completely covering it. 1

A thin, rupturable diaphragm 7 of, for example, 1 to 2 mil thick lead foil, is held in spaced relation to the bridgewire 5 by an electrically insulating spacer 8 and a retaining spacer 9, which may be of metal. The diaphragm 7 may be coated on the side opposite the bridgewire 5 with a very fast-burning pyrotechnic composition 10, composed of any such as are knownin the art. A main pyrotechnic charge 11, which may be any of those common in the art or composed as described in copending application Serial Number 60,134,, filed October 3, 1960, is placed above the diaphragm'7. A cl0- sure 12, commonly of metal foil, retains and seals the main charge 11.

The compositions of the present invention, which are applied as a coating '6 to the bridgewire 5, are composed of finely divided particles, 44 microns or smaller in size, of a secondary explosive, suchjas pentaerythritol tetranitrate (PETN), bound with a water soluble inorganic 'alkali silicate, such as sodium silicate. The silicate com- A preferred composition which has been found suitable for. use in EBW squibs suchas that shown in the drawing is prepared by mixing 30 parts by weight of purified PETN, finer than 325 mesh, With 70 parts by weight of a solution of sodium silicate in water. The silicate solution for this example has a hydrometer value of 40' degrees B-aum, a density of 1.38 grams per cubic centimeter, and a ratio of Na O to SiO of 1:3.25. The

- mixture is coated onto the bridgewire 5 and the insulator 3 to the desired thickness, and the partially assembled squib is then heated in an oven to drive oif the water. The final composition ;on a dry weight basis, after evapsilicate. V

In practice, the ratio of silicate binder to secondary explosive may be adjusted in a manner that gives optimum balance between the mechanical properties needed for adhesion and strength, and the chemical properties needed for rapid reaction. Depending upon the effect desired and the electrical energy provided for actuation, compositions ranging from 40 to 70% secondary explosive bound with 30 to 60% silicate binder, on a dry weight basis, may-be employed.

A number of secondary explosives other than PETN have been found suitable for use in the compositions of the present invention. Such materials as, for example, cyclotrimethylenetrinitramine (RDX) may be employed.

A preferred silicate binder is composed of sodium silicate (water glass) which has a formula designation of Na O-(SiO where x is approximately 3. Alternate names and formulae for the primary ingredient of the binder are sodium tetrasilicate, Na Si O or water soluble silicates, such as sodium metalsilicate, Na SiO or potassium disilicate, K Si O I The coating compositions of the present invention per-' 7 form several functions. They act as both electrical and thermal insulators, as heat sinks, and as radiators to prevent localized overheating due to accidental passage of electrical energy through the bridgewire. In addition, when the bridgewire is exploded upon deliberate actuation, the coating contributes to the explosive energy released, rupturing the diaphragm and igniting the pyrotechnic charges.

Uncoated bridgewires, when used in a squib such as is shown in the drawing, frequently 'do not release sufficient energy to rupture the diaphragm and ignite the pyrotechnic charges. If the thickness of the diaphragm is reduced, or if the diaphragm is spaced in closer proximity to the bridgewire, the squib may he accidentally actuated by passage of suflicient electrical energy through the bridgewire to heat it to incandescence or to fuse it. Accidental actuation is even more likely in squibs in which a diaphragm is not used. Exploding bridgewire squibs, employing bridgewires coated with compositions it should not be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit of the invention or the scope of the subjoined claims.

What is claimed is:

1. In a deflagrating squib of the exploding bridgewire type, the combination with a bridgewire of a limitedly detonating coating applied to said bridgewire, said coating consisting of a finely divided secondary explosive in .a Water-soluble inorganic silicate binder.

2. The combination, as in claim 1, wherein said secondary explosive consists of particles that are 44 microns or smaller in size.

3. The combination, as in claim 1, wherein said secondary explosive is pentaerythritol tetranitrate.

4. The combination, as in claim 1, wherein said secondary explosive is cyclotrimethylenetrinitramine.

5. The combination, as in claim 1, wherein said silicate binder consists of at least one silicate selected from the class consisting of water-soluble sodium silicate compounds.

6. The combination, as in claim 1, wherein said silicate binder consists of a mixture of silicates selected from the class consisting of water-soluble sodium silicate compounds.

7. The combination, as in claim 1, wherein said silicate binder consists of at least one silicate selected from the class consisting of water-soluble potassium silicate compounds.

8. The combination, as in claim 1, wherein said silicate binder consists of a mixture of silicates selected from the class consisting of water-soluble potassium silicate compounds.

9. The combination, as in claim 1, wherein said silicate binder consists of a mixture of silicates selected from the class consisting of water-soluble sodium and potassiumsilicate compounds.

. 1 0. A'coating for an exploding bridgewire in a defilagrating squib, said coating consisting of a finely divided secondary explosive in awater-soluble inorganic silicate binder.

11. A coating, as in claim 10, wherein said secondary explosive consists of particles that are 44 microns or smaller in size.

12. A coating, as in claim 10, wherein said secondary explosive is pentaerythritol tetranitrate.

13. A coating, as in claim 10, wherein said secondary explosive is cyclotrimethylenetrinitramine.

14. A coating, as in claim 10, wherein said silicate binder consists of at least one silicate selected from the .class consisting of'water-soluble sodium silicate compounds.

15. A coating, as in claim 10, wherein said silicate binder consists of a mixture of silicates selected from the class consisting of water-soluble sodium silicate compounds.

16. A coating, as in claim 10, wherein said silicate binder consists of at least one silicate selected from the class consisting of Water-soluble potassium silicate compounds.

17. A coating, as in claim 10, wherein said silicate binder consists of a mixture of silicates selected from the class consisting of water-soluble potassium silicate compounds.

18. A coating, as in claim 10, wherein said silicate binder consists of a mixture of silicates selected from the class consisting of water-soluble sodium and potassium silicate compounds.

References Cited in the file of this patent UNITED STATES PATENTS 2,926,566 Atkins et al. Mar. 1, 1960 2,982,639 Gey May 2, 1961 2,992,087 I Fassnacht July 11, 1961 3,040,660 Johnston June 26, 1962 3,048,507 Zebree Aug. 7, 1962 3,054,258 Marti Sept. 18, 1962 3,059,576 Haefner Oct. 23, 1962 

10. A COATING FOR AN EXPLODING BRIDGEWIRE IN A DEFLAGRATING SQUIB, SAID COATING CONSISTING OF A FINELY DIVIDED SECONDARY EXPLOSIVE IN A WATER-SOUBLE INORGANIC SILICATE BINDER. 